In a turbo-machine, it is known to make use of one or more stator vane stages for adjusting the stream and the flow direction of the gas passing through the compression section as a function of the operating speed of the turbo-machine. These stator vane stages comprise a plurality of vanes (also known as variable-pitch vanes) that are capable of pivoting about their axes connecting them to the stator so that their pitch angle can be modified as a function of the operating speed of the turbo-machine.
Known devices for controlling the pivotable vanes of a turbo-machine usually comprise a plurality of pivotable vanes distributed in azimuth over at least 90° (ninety degrees of angle) around the axis of the turbo-machine, said pivotable vanes being oriented substantially radially relative to the axis of the turbo-machine, and a control ring portion for controlling the pivoting of the vanes, each vane being connected to the control ring portion by a link, the control ring portion being held around the axis of the turbo-machine by the set of links.
In turbo-machines that have an axial compressor, the vanes are oriented radially around the axis of the turbo-machine and they are pivotable about respective radial axes. It should be observed that the term “oriented substantially radially” covers any configuration in which the radial axes about which the vanes are pivotable form an angle lying in the range 45° to 90° with the axis of the turbo-machine.
Furthermore, it should also be observed that the term “ring portion” is used to cover equally well a complete ring or merely a portion of a ring.
The ring portion is generally controlled by an actuator that causes it to turn about the axis of the turbo-machine, in one direction or the other. The movements of devices of that type are complex and very precise, such that if precision clearances are not complied with, the device can become statically indeterminate (i.e. it jams). In particular, this implies drastic constraints on the positioning and centering of the ring portion relative to the axis of the turbo-machine. Thus, a small departure from these positions leads very quickly to high levels of stress throughout the control device, and possibly to it becoming jammed. This problem of centering (or more generally of positioning) is made worse by thermal expansion differences between the various parts of the device.